Ionization of nitrotriphenylmethanes. Remarkable kinetic evidence for steric inhibition to resonance and F-strain

Abstract

Rate and equilibrium data for the reversible deprotonation of 2,2′,2″,4,4′,4″-hexanitro-, 2,2′,4,4′,4″-pentanitro-, 2,4,4′,4″-tetranitro-, 4,4′,4″,-trinitro-, 4,4′-dinitro- and 4-nitro-triphenylmethanes (2a–f) by hydroxide ion have been measured in various H₂O–dimethyl sulphoxide (DMSO) mixtures at 25 °C. The increase in acidity (pK_a) brought about by the introduction of a first p-nitro group in triphenylmethane (2g) to give (2f) is very large and equal to about 12 pK units while the acid-strengthening influence of each of the second and third p-nitro groups is only of the order of 2 pK units. This suggests that only one p-nitrophenyl ring of the 4,4′-dinitro- and 4,4′,4″-trinitro-triphenylmethyl anions [(C-2e), (C-2d)] is in a favourable position for effective conjugation with the exocylic sp² carbon atom of these carbanions at any given time. Accordingly, the effects exerted by the additional p-nitrophenyl ring(s) are mainly inductive in nature. Support for these ideas is the observation that the increases in acidity observed on going from (2f) to (2e) to (2d) are essentially the result of the corresponding increases in the kinetic acidity (k_–p^H₂O). Addition of a first o-nitro group to (2d) to form (2c) further increases the kinetic acidity but it also results ina large decrease in k_–p^H₂O, consistent with a preferential stabilization of the corresponding 2,4,4′,4″-tetranitrotriphenyi-methyl carbanion (C-2c) by the 2,4-dinitrophenyl ring. Significantly the introduction of the second and third o-nitro groups enhances the thermodynamic acidity while it decreases markedly the k_p^OH and k_–p^H₂O values. It is suggested that these anomalous variations are the reflection of unfavourable steric interactions arising from the accumulation of o-nitro groups in the triphenylmethane system.